Spatial localization of holes in La^sub 2 - x^Sr^sub x^CuO^sub 4

The parameters of the tensor of the electric field gradient (EFG) in cation sites of the La^sub 2 - x^Sr^sub x^CuO^sub 4^ lattice have been determined by the method of emission Mössbauer spectroscopy on ^sup 57^Co(^sup 57m^Fe), ^sup 67^Cu(^sup 67^Zn), ^sup 67^Ga(^sup 67^Zn), and ^sup 155^Eu(^sup 155^Gd) isotopes. There is no quantitative agreement between the calculated (the pointcharge model) and experimental values of the main component of the tensor EFG V ^sub zz^, which is explained by the absence of the reliable data on the Sternheimer coefficients for Fe^sup 3+^, Zn^sup 2+^, and Gd^sup 3+^ ions. Based on the comparison of the calculated and experimental dependences of V ^sub zz^ on x it was shown that the holes appearing during the substitution of La^sup 3+^ for Sr^sup 2+^ are localized preferably on the oxygen atoms that are in the same plane as the copper atoms, which is in agreement with the model discussed in the literature and assumes that the mechanism responsible for the high-temperature superconductivity of solid solutions La^sub 2 - x^Sr^sub x^CuO^sub 4^ is the interaction between the conductivity electrons and two-atomic two-electron centers with negative correlation energy.[PUBLICATION ABSTRACT]